I am trying to pick which downpipe to use with the stock cat. I first wanted the HKS downpipe, same diameter as cat, but is too expensive. Buschur looks good.
Then I hear the Helix bolts to the stock cat and is cheaper but is 3". I have seen others bolt the Helix to the stock cat.
I guess my question is, which will I have more power with using the stock cat? 3" definitely more power, but with the stcok cat would I be better off with the 2.5 since it is the same size as cat? Or will I be better with 3" even though it will neck down? Thank you.

 

The Helix is True 3", it does not neck down. I have it and am VERY happy with it combined with a testpipe and catback..

 

I know it is 3". That isn't my question. I could buy the Helix, but the exhaust tract will neck down when it gets to the stock cat, so am I better off with a 2.5 inch?

 

Cheapest is Espelir or Megan Racing DPs.
Go 3 inch it will still be better even with stock cats.

 

Go for the 3", it will bolt up to the 2.5" no problems.

 

I have the Hks dp. Its 2.75 great for spool up, ok on top end. If you are going to keep the stock turbo its pretty good. If you are going to change the turbo go 3inches. Sometimes bigger is not always better.